Assuming no thermal limitations, power transfer limits for a power system frequently arise from concerns about transient instability or voltage instability in the event of a contingency. There are also concerns regarding steady-state instability. In order to quantify these potential instabilities a knowledge of the power system's dynamic characteristics is necessary. Existing techniques used to provide estimates of a power system's dynamic characteristics, and hence power transfer limits, are based on mathematical dynamic modelling studies which are subject to significant uncertainties. Hitherto system engineers have had to build in large factors of safety, effectively discounting the safe power transfer capacity by a considerable margin, and thus unduly limiting the power which can be transferred or requiring excess investment in capacity.
It has previously been proposed to use a “signal energy” approach to the setting of power transfer limits, based on the observation that “signal energy” increases (and damping deteriorates) asymptotically as the power flow increases.
However, these proposals suffer from the facts that:    (a) they rely solely on mathematical dynamic modelling, with the attendant problems discussed above    (b) the use of “signal energy” without splitting this quantity into frequency components obscures the nature of the problem    (c) the relationship between signal energy and/or damping and MW power flow is not, in practice, at all uniform.